It is well-known that in a strongly focused laser beam having an approximately Gaussian intensity profile, radiation pressure scattering and gradient force components are combined to provide a point of stable equilibrium located close to the focus of the laser beam. Scattering force is proportional to optical intensity and acts in the direction of the incident laser light. Gradient force is proportional to the optical intensity and points in the direction of the intensity gradient.
This effect is utilized in a so-called optical tweezer or optical trap since the optical gradient forces in a focused light beam trap a small micro-object at the focal point of the light beam. The micro-object is typically immersed in a fluid medium whose refractive index is lower than that of the micro-object. The optical tweezer technique has been generalized to enable manipulation of reflecting, absorbing and low dielectric constant micro-objects. Typically, a Gaussian beam is used for trapping of a micro-object with a refractive index that is higher than the refractive index of its surroundings while a donut beam is used for trapping of a micro-object with refractive index that is lower than the refractive index of its surroundings.
In U.S. Pat. No. 4,893,886, an optical trap for biological micro-objects is disclosed wherein biological micro-objects are kept in a single-beam gradient force trap using an infrared laser.
In U.S. Pat. No. 6,055,106, an apparatus for manipulating micro-objects is disclosed that comprises a diffractive optical element for receiving a light beam and forming a plurality of separate light beams, each of which is focused to form a separate optical trap or tweezer.